Crystal Structures ofN-Aryl-N′-4-Nitrophenyl Ureas: Molecular Conformation and Weak Interactions Direct the Strong Hydrogen Bond Synthon

Hydrogen bond competition was studied in 21 X-ray crystal structures of N-X-phenyl-N′-p-nitrophenyl urea compounds (X = H, F, Cl, Br, I, CN, C≡CH, CONH2, COCH3, OH, Me). These structures are classified into two families depending on the hydrogen bond pattern: urea tape structures contain the well-known α-network assembled via N−H···O hydrogen bonds; however, in nonurea tape structures the N–H donors hydrogen bond with NO2 groups or solvent O acceptor atoms. Surprisingly, the urea C═O hardly accepts strong H bonds in nonurea type structures sustained by urea···nitro and urea···solvent synthons. The carbonyl group accepts intra- and intermolecular C−H···O interactions. The molecular conformation and H bonding motifs are different in the two categories of structures: the phenyl rings are twisted out of the urea plane in the tape motif, but they are coplanar in the nonurea category. Even though hydrogen bond synthon energy and urea carbonyl acceptor strength favor the N−H···O tape structure, the dominant patt...